Identifying Modifiers of Anticipation in Myotonic Dystrophy Type-1 Abstract Many neurodegenerative conditions are caused by expansion of trinucleotide repeats (TNRs). Myotonic dystrophy type-1 is caused by a CTG repeat expansion in the 3'UTR of the DMPK gene. This progressive disorder affects nearly every system in the body, causing progressive weakness, cardiac arrhythmias, intellectual impairment, and respiratory failure. Myotonic dystrophy type-1 is a classic example of genetic anticipation whereby progressively more severe symptoms are seen in successive generations due to increasingly large CTG repeat expansions during germline transmission. In the most striking example of this, mothers with adult onset myotonic dystrophy may have a child with congenital onset myotonic dystrophy (CDM). The exact mechanism of repeat expansion in myotonic dystrophy is complex, with DNA repair, replication, recombination and even transcription implicated in the unstable nature of triplet repeats. This proposal aims to better understand the mechanism of anticipation through: 1) Identification of variants in DNA repair and replication genes in mothers who have a child with the most severe form of myotonic dystrophy, CDM, 2) Assessing the pathogenicity of these variants in a cell model system for precise measurement of difference in repeat expansion rates. Firstly, this proposal will perform whole exome sequencing using a cohort highly enriched for anticipation, those mothers who have had a child with congenital myotonic dystrophy. Currently, 45 families within this cohort have provided DNA. Whole exome sequencing will be performed to identify variants in DNA damage response genes. Secondly, variants will be assessed for pathogenicity in a cell model system. In this system, the CTG repeat length is correlated with the expression of a fluorescent reporter gene. The longer the CTG repeat, the lower the fluorescence. Variants identified in Aim 1 will be introduced into this system to assess for their effect on the CTG repeat length. Two variants in the DNA damage response pathway, CHEK1 and FANCM, which are suspected to have altered the mutation rate in a family with Duchenne Muscular Dystrophy will also be assessed. At the completion of this proposal, we would hope to better understand the mechanism of anticipation, and repeat expansion in general. Such knowledge may provide therapeutic targets to modulate repeat expansion.